A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”—direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Conventionally known precision sensors, such as interferometers, may provide accurate position measurements. However, the accuracy of conventional interferometers is limited by disturbances in the air through which the radiation beam of the interferometer passes. Such disturbances may include air turbulence and thermal variations. Accordingly, the accuracy of conventional interferometers can only be increased by minimizing such disturbances. However, minimizing such disturbances, such as by introducing delays in order to allow the air turbulence to decrease and/or to allow the temperature of the air to settle to within a required value, reduces the throughput of a lithographic apparatus and, accordingly, increases the cost of using the apparatus.
One way to reduce the disturbances is to limit the length of the air path in the measurement beams. This may be done by using large two dimensional Grid Plates above the substrate table with substrate table mounted encoders or by using an array of detectors. Both solutions require large volume and cost.
Furthermore, any measurement system may only occupy a limited volume of space within the lithographic apparatus.